Selective firing system



April 19, 1966 w. T. BELL 3,246,707

SELECTIVE FIRING SYSTEM Filed Feb. 17, 1964 5 Sheets-Sheet l III AJV/I I I ma INVENTOR.

ATTORNEY ZMVZQLM A ril 19, 1966 W. T. BELL 3,246,707

SELECTIVE FIRING SYSTEM Filed Feb. 17, 1964 5 Sheets-Sheet z INVENTOR.

A ril 19, 1966 w. T. BELL SELECTIVE FIRING SYSTEM 5 Sheets$heet 5 Filed Feb. 17, 1964 United States Patent 3,246,707 SELECTIVE FIRING SYSTEM William T. Bell, Houston, Tex., assignor to Schlumberger Well Surveying Corporation, Houston, Tern, a corporation of Texas Filed Feb. 17, 1964, Ser. No. 345,274

8 Claims. (Cl. 175-454) This invention relates to shaped-charge perforating systems for perforation of oil wells, and more particularly to apparatus for selectively firing individual shaped charges of a perforating tool.

There is a trend in well completion techniques from closely-spaced multiple perforating at a selected depth toward single-shot perforating at various selected depths in a well. To provide single perforations at various depths, apparatus having a number of shaped charges is lowered into a Well to a particular depth where a perforation is to be made. One of the charges is detonated and, thereafter, the apparatus is repositioned to the next level where another perforation is desired. A second charge is then fired and this is continued until all of the desired perforations have been made.

It is recognized by those skilled in the art that when shaped charges are to be individually and selectively detonated, provisions must be made to detonate each shaped charge individually as well as to electrically isolate them from each other to prevent accidental detonation or machine gunning of the remaining shaped charges at the same time. Accordingly, a well-designed selective-firing shaped charge perforating carrier is generally comprised of a series of tandemly-arranged pressure-sealed hollow charge housings separated from one another by individual firing-head subs or housings, with each charge housing and its firing head constituting a single perforating unit. An explosive charge comprised of a single shaped charge and its associated detonating train is enclosed in each of the charge housings to provide a protective air cushion around the shaped charge. This air cushion prevents the explosive forces from either deforming or rupturing the carrier or charge housing, in which case the carrier is not re-usable or may be so badly deformed that it cannot be retrieved from the well bore.

The detonating train for a shaped charge includes a length of detonating cord and an electrically-fired blasting cap connected through an arming device in the firing-head sub or housing to an electrical firing circuit which leads to a voltage source at the surface of the ground. These arming devices can be considered as normally-open switches which are actuated in response to the firing of a shaped charge immediately adjacent to the device. Upon firing of a shaped charge, the arming device for the next adjacent perforating unit is operated to connect its related blasting cap into the electrical firing circuit to ready hat shaped charge for firing as needed. Thus, as each shaped charge is fired, it arms the next adjacent perforating unit by connecting its blasting cap into the firing circuit.

In general, the electrical firing circuits used with selectively-fired shaped charge systems fall into one of three types. In one type, the firing circuit is a series-parallel arrangement of blasting caps having filament igniters requiring different intensities of current for their ignition and individual arming devices for each blasting cap except for the one to be fired first. The igniters are serially connected in one side of the firing circuit, with the arming devices being in parallel with each other for electrically connecting each igniter in turn to the return side of the firing circuit. Initially, the igniter of the first-fired blasting cap is the only one connected to the return side of the firing circuit and this igniter requires the lowest level of ignition current. The other igniters are successively arranged with each succeeding igniter requiring a progressively greater ignition current before it will fire. Thus, when a shaped charge is detonated, the resulting explosion closes the arming device of the next perforating unit to connect its associated blasting cap into the return path of the firing circuit.

A second type of conventional firing circuit employs a parallel circuit of equal-sensitivity blasting caps with each filament igniter, except for the first-fired cap, being in series with a polarity-sensitive means, such as a rectifier or diode, and a normally-open switch portion of an arming device. In a perforating tool, these rectifiers are alternately oriented with every other rectifier being arranged to pass only voltage of one polarity from the main conductor and the intermediate rectifiers being arranged to pass only voltage of the opposite polarity from the main conductor. With this arrangement, only the first blasting cap will be detonated when voltage of a first polarity is applied to the main conductor. Even though the resulting explosion closes the next arming device and connects the next blasting cap to the main conductor, this second blasting cap cannot be detonated until voltage of the opposite polarity is applied to the main conductor. Thus, to detonate each shaped charge, it is necessary that the polarity of the firing voltage be reversed each time.

A third commonly-used firing circuit employs a rotary selector switch for sequentially applying a firing voltage separately to each blasting cap as required. These firing circuits are generally used with a suspension cable, called a inonocable, which includes a single main electrical conductor protected by a conductive armored sheath which acts as the other conductor or return path of the firing circuit.

It will be appreciated that if an intermediate unit is inadvertently fired out of sequence, the well will be perforated at a point removed from the desired depth of perforation without the operator being aware of this. Consequently, all successive shots will unwittingly be olfdepth by whatever distance the intermediate unit is above the lower unit which was to be fired first. Thus, if well liquids do leak into a carrier, it is best that the firing circuit for the perforating units above the leaking unit cannot be rendered operative by hydrostatic pressure so these intermediate units cannot be fired out out of sequence.

It is also preferred to progressively disconnect the main conductor of a parallel-arranged firing circuit from each perforating unit after it is fired. It will be appreciated that if this is not done, the main conductor may be shorted to the carrier in a lower unit which will prevent the subsequent application of full firing voltage to a perforating unit thereabove. Thus, a well-designed arming device for such a firing circuit should progressively disconnect the main conductor when a perforating unit is fired as it simultaneously connects the main conductor to the next perforating unit to be fired.

Arming devices of certain prior-art units which depend upon engagement of an isolated electrical contact with the carrier housing to complete the firing circuit are often unreliable. For example, in those firing circuits employing variable-sensitivity blasting cap igniters in series with each other, an arming device of this type may not establish a sound electrical connection when it is first engaged with the carrier because of an imperfect contact or an oxide film on the carrier or housing. In this event, the electrical resistance added to the circuit may require a substantial increase of voltage to reach the particular current level neededto fire the next blasting cap. Accordingly, it frequently happens that as the voltage is being increased to bring the current to the proper level for firing the next cap, an arc is formed across such an imperfect contact which suddenly establishes a more perfect contact and reduces the contact resistance. This sudden lowering of the total circuit resistance will naturally cause the current to increase rapidly and often causes a current surge of suflicient intensity to fire one or more successive caps simultaneously.

Other commonly-experienced difliculties include malfunctions which occur when interconnecting wires within the housings become shorted or broken during assembly of the perforating tool and the failure of complicated switching mechanisms to withstand the rigorous conditions found in the oil field. It will be understood, therefore, that an optimum selective-firing system for shapedcharge perforators should (1) be reliable and safe both in loading. and in use, (2) include a positive firing selectivity which eliminates the possibilty of machine gunning, (3) positively disconnect the main conductor from each perforating unit after it is fired, (4) withstand high pressures encountered in a well bore, (5) be simple in construction for ease in arming and loading of individual charges, and (6) deactivate the firing circuit above it should well fluids leak into the tool.

It is, therefore, an object of the present invention to provide a reliable mistake-proof system for selectively and separately arming a plurality of shaped charges in a predetermined sequence.

It is another object of the invention to provide a selective firing system for a plurality of perforating units which may be embodied in an easily assembled, ruggcdly constructed, substantially mistake-proof perforating apparatus capable of providing safe and reliable operation under service conditions in the field.

It is a further object of the invention to provide a positively acting arming switch which fails safe in the event that well fluids leak into an adjacent perforating unit.

It is yet another object of the invention to provide a positively acting arming switch which will be actuated only when the preceding shaped charges have been detonated in a predetermined sequence.

It is an additional object of the invention to provide arming switches of a rugged construction which are inexpensive to manufacture and will always make a positive electrical contact whenever they are closed.

It is a further object of the invention to provide positively sealed arming switches to prevent leakage of well fluids into the next perforating unit of a perforating tool.

These and other objects of the present invention are obtained by apparatus which includes various embodiments of arming switches for use within firing-head subs or housings adapted to be coupled to a charge housing, which assembly constitutes a single perforating unit. An explosive charge, comprised of an electrically-fired blasting cap connected to a shaped charge by a length of blast ing cord, is disposed within a chamber in the charge housing and is arranged for detonation upon application of a firing voltage to the blasting cap. The blasting cap is serially connected to the arming switch so that the cap cannot be fired until the arming switch is positively closed.

In general, the arming switches employing the principles of the present invention include a piston member which is uniquely sealed relative to the charge housing to prevent entry of well fluids around the piston member into adjacent charge housings. Each switch is preferably positively retained in a first off position by a yieldable restraining member, such as a shear pin or spring, which holds the switch open until the shaped charge in the charge housing immediately below the switch is detonated. The dynamic explosive forces produced by either one or both the concussion of the explosion and the sudden inrush of well fluids into the housing act against the outwardlydirected face of the piston to impel the piston inwardly and close the arming switch. The arming switch can be either an integral part of the piston member or a separate switch with an operator arranged for actuation by the piston member when it travels to a second position. In some embodiments of the present invention, the hazard that an arming switch might be prematurely closed by undetected leakage of well fluids into the perforating unit beneath the switch is positively eliminated by providing a small leakage passage across the piston member. Thus, should well fluids leak into a unit, the pressure across the piston will be equalized to prevent the hydrostatic pressure from closing the arming switch immediately thereabove and consequently allow only those shaped charges below that unit to be fired.

Additional objects and advantages of the invention will become apparent from the following detailed description of representative embodiments thereof, taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates schematically a series-paralleled firing circuit used in conjunction with the arming switches of the invention;

FIG. 2 shows in detail one embodiment of an arming switch used in the firing circuit of FIG. 1;

FIG. 3 shows selective-firing heads using the switch of FIG. 2 coupled to charge housings in an assembly of perforating units;

FIG. 4 shows in detail a second embodiment of an arming switch of the invention which may be used in either the firing circuit of FIG. 1 or FIG. 5;

FIG. 5 illustrates schematically an alternate paralleled firing circuit in which the arming switches of either FIG. 4 or FIG. 7 may be employed;

FIG. 6 shows the arming switch of FIG. 7 mounted in selective firing heads coupled'to charge housings for assembling a perforating tool; and

FIG. 7 shows an alternate embodiment of an arming switch in accordance with the principles of the invention which may be employed in the firing circuits of FIGS. 1 and 5.

Turning now to FIG. 1, a series-paralleled firing circuit for a perforating apparatus 10 is schematically illustrated which may be employed for selective detonation of a series of shaped charges (not shown) in a carrier 11. The carrier 11 is suspended in a well bore by a monocable 12 which has a single inner electrical conductor 13 and an outer armor sheath 14 for the electrical return path.

The firing circuit includes a remote electrical source, such as a battery 15, which is located at the surface of the ground and has one pole connected at 16 to the electrical return path and the other pole connected to the central conductor 13 of the monocable 12. A potentiometer 17 and ammeter 18, in series with a control switch 19, are employed to adjust and control the amount of electrical current flowing through the firing circuit.

The blasting cap filament igniters 20, 21 and 22 for the detonation of separate shaped charges (not shown) are serially connected to the main conductor and successively arranged so that each igniter, in turn, requires a progressively greater ignition current, with the igniter 20 requiring the lowest ignition current being in the lowermost perforating unit 23 which is to be fired first. The lower end of the igniter 20 in the lowermost perforating unit 23 is connected to the housing 24 of carrier 11 which is, in turn, connected to the armor sheath 14 so as to complete the return path of the firing circuit back to the voltage source 15.

Paralleled normally-open arming switches 25, 26 are respectively disposed between the lowermost unit 23 of the carrier 11 and the units 27, 28 immediately thereabove. These arming switches 25, 26 are so arranged that whenever the shaped charge immediately 'below an arming switch is fired, the resulting dynamic explosive forces from the concussion and fluid pressure will close that switch to connect the next igniter to the return path of the firing circuit. Thus, it will be appreciated that whenever the lowermost or first perforating unit 23 is fired, the resulting dynamic explosive forces from the concussion of the explosion and the inrush of well fluids into the charge housing of that unit will actuate the arming switch 25 immediately thereabove to connect the lower end of the igniter 21 in the unit 27 immediately thereabove to the electrical return path. Then, whenever it is desired to fire the second unit 27 in the apparatus or tool 10, the control switch 19 is again closed and potentiometer 17 is readjusted to increase the electrical current to the level necessary to fire the next blasting cap igniter 21. When this second unit 27 is fired, the explosive forces resulting therefrom will actuate and close the arming switch 26'of the next unit 28 immediately thereabove; and the cycle is repeated so long as there are yet shaped charges to be fired.

In FIG. 2, one embodiment of an arming switch 29 constructed in accordance with the principles of the pres ent invention is shown in detail. This arming switch 29 is carried within a firing-head sub or housing 30 adapted for connection between adjacent charge housing sections in the perforating apparatus or tool. A piston member 31 disposed within a longitudinal bore 32 extending through the firing head 30 is arranged to travel upwardly from an inoperative first position (as shown in FIG. 2) to an operative second position, which is longitudinally spaced relative to the first position, in response to the detonation of the shaped charge in the perforating unit immediately below firing head 30.

A sealing member, such as an O-ring 33 received in a peripheral groove 34 around the piston member 31, is seal-ingly engaged with the walls of the bore 32 through the firing head. Accordingly, whenever the shaped charge immediately below the arming switch 29' is fired and closes the switch, O-ring 33 prevents well fluids which rush into the lower charge housing from entering the adjacent charge housing and rendering its explosive charge inoperable.

One or more resilient, finger-like members 35 are provided around the lower face 36 of the piston member 31. These fingers 35 project angularly downwardly and outwardly from the piston member 31 and are arranged for engagement with a stop member, such as a groove or an annular ring 37, in the wall of the bore 32 and. longitudinally spaced below the piston 31. These fingers 35 assist the O-ring in holding the piston 31 in position. If desired, a light compression spring 38 may be disposed between a shoulder 39 in the bore 32 above the piston 31 and the upper face 40 of the piston to doubly ensure that the piston remains in its inoperative position until it has been explosively impelled to its operative position.

The piston 31 is provided with a longitudinal bore 41 which extends through the piston. An insulating member, such as a tubular electrically-nonconductive sleeve 42, is received in this bore and an electrically-conductive member, such as a screw 43, extending through the sleeve 42 is threadedly engaged with an enlarged electricallyconductive contact member or head 44 which is also insulated from the upper face 40 of the piston by an insulating washer 45. These conductive members 43, 44 comprise one contact of the arming switch as well as provide a conductive path bet-ween adjacent perforating units. The other contact of the arming switch is provided by the inwardly-directed shoulder 39 in the bore 32 of housing 30 above the piston. Thus, whenever piston member 31 is explosively impelled to its operative or second position, enlarged contact member 44 on the upper face of the piston will be driven into engagement with shoulder 39 to make an electrical connection from conductive member 44 carried by the piston to the housing 30. It is preferable that this enlarged contact member 44 be of a relatively-soft electrically-conductive material to ensure that a good electrical contact will be established whenever the piston 31 is driven into its second position.

The resilient fingers or restraining membes 35 are of a suflicient length so that as the piston member 31 is explosively impelled from its first or inoperative position to its second or operative position, the stop ring 37 will momentarily compress the fingers inwardly until they clear the stop ring 37 and are then free to spring out- Wardly into engagement above the stop ring 37. Accord ingly, the free ends 46 of members 35 will engage the upper side of the stop ring 37 to retain the piston member 31 in its operative position which holds the arming switch 29 closed.

So that a series electrical connection may be made from one arming switch to the next, a tapped bore 47 is provided at the upper end of the screw 43 passing through the piston member 31 and a second screw 48 is threadedly received by this tapped bore 47. Thus, it will be appreciated that electrical conductors 49, 50 may be connected to opposite ends of the screw 43 passing through the piston member 31 to provide a continuous electrical path through the piston member.

The novel switch of FIG. 2 is shown operatively assembled in FIG. 3. As shown in this figure, a single perforating unit 51, comprised of a charge housing 52 and a selective-firing head sub or housing 53, is tandemly coupled between two other identical perforating units 54, 55. The upper end of charge housing 52 is connected to the lower end of a second selective-firing housing 56 which is a part of the perforating unit 54 thereabove. At the lower end of selectivefiring head 53, the upper end of the charge housing 57 of the lower perforating unit is partially shown.

The intermediate perforating unit 51 shown in full carries within its inner bore 58 an explosive charge 59 comprised of a shaped charge 60, a blasting cord 61 and an electrically-fired blasting cap 62 depending from the lower end of the blasting cord. The shaped charge is mounted within the bore 58 of the charge housing 52 opposite an explosion port 63 in the wall of the charge housing 52. A port closure member 64 is sealingly received in the explosion port 63 in the well-known manner. The blasting cap 62 is disposed within the bore 65 of the firing head 53 adjacent to and proximately spaced from an access port 66 in the side of the selective-firing head 53. A closure member 67 is sealingly received within the access port 66 in a manner similar to the closure member. A switch 68, such as that illustrated in FIG. 2, is shown in the lowermost selective-firing head 53 as being in its second or operative position while the arming switch 69 in the upper selective-firing head 56 is still in its inoperative position. Thus, it will be understood that the explosive charge which had been carried in the perforating unit 55 immediately below the intermediate perforating unit 51 has already fired which actuated the arming switch 68 of the intermediate perforating unit 51.

It will be appreciated that access port 66 provides a ready and convenient access to the electrical terminals or contacts 76 of blasting cap 62. Thus, the conductor 71 from the upper arming switch 69 and conductor 72 to the lower arming switch 68 need not be connected to the blasting cap 62 until after the housings have been completely assembled. This will enable the housings to be freely rotated during assembly with the conductors extending out of the access port to ensure that they do not become entangled in the housing threads.

It will be understood that the arming switch 29 of FIG. 2 will close should well fluids leak into the perforating unit below it. This does not present any problem of inadvertently firing off-depth, however, since a different intensity of current is required to fire each of the blasting caps used in conjunction with this switch.

The preferred mode of operation with this switch 29 is to select blasting cap igniters of suificiently differing sensitivities that the current required to fire one igniter is considerably less than that required to fire the next successive igniter. In actual practice, therefore, a skilled operator will not increase the current substantially over the level of sensitivity required to fire a particular cap.

Thus, should leakage in one perforating unit close the arming switch of the next unit, the well liquid generally renders that blasting cap inoperative so the operator will realize that something is not functioning properly when the current level reaches the required intensity without detonating that particular charge. Accordingly, the skilled operator will immediately open the firing circuit control switch and retrieve the perforating tool.

It will be appreciated that the arming switch of FIG. 2 provides an easily-constructed, rugged and durable unit which is inexpensive but still ensures a low-resistance and positive electrical contact, as well as prevents leakage of well fluids into adjacent perforating units.

In FIG. 4, a second embodiment of a switch device 73 employing principles of the present invention is shown. This switch 73 is enclosed in a selective-firing head or housing 74 and includes an explosively-impelled plunger member 75 and a self-contained electrical switch 76 operatively arranged and disposed relative to the plunger member 75 so as to be operated thereby whenever the plunger member 75 is driven from an inoperative first position to an operative second position.

Although the firing-head sub or housing 74 may be coupled to the adjacent charge housings by threads as in the embodiment of FIG. 2, it is preferred to construct the firing-head 74 as shown in FIG. 4. The tubular head 74 has an enlarged diameter outer flange portion or shoulder 77 midway between its ends with each of the ends being sized for snug reception into the open end of the adjacent charge housings 78, 79. Sealing members, such as O-rings 80, 81 carried in peripheral grooves 82, 83 around each firing-head end portion sealingly engage the inner walls of the charge housings 78, 79 to prevent leakage of well fluids into the perforating units. A plurality of laterally-extending screws 84, 85 secure each of the charge housings 78, 79 to the selective-firing head 74.

The tubular selective-firing head 74 is provided with a longitudinal bore 86 extending therethrough and arranged for reception in one end thereof of the arming switch case 87. Sealing members, such as O-rings 88, encircling one end of the switch case 87 are provided to fluidly seal the switch case relative to the selective-firing head 74 and a set screw 89 through the selective-firing head 74- secures the switch case 87 in place.

Plunger member 75 is received within a longitudinal bore 90 through the switch case 87 and arranged for travel upwardly from a first position to a second position longitudinally spaced relative to the housing. As seen in FIG. 4, the longitudinal bore 99 through the switch case 87 includes at least an enlarged bore portion 91 at its lower end and a reduced bore portion 92 at its upper end. The plunger member 75 itself is comprised of a piston portion 93 adapted for reception in the enlarged bore portion 91 and a shank portion 94 which is received in and extends out of the reduced bore portion 92 a sufficient distance so that it will engage and actuate the actuator or operator 95 of the switch 76 whenever the plunger member 75 travels upwardly from its first position to its second position.

A restraining member, such as a shear pin 96, is connected between the switch case 87 and plunger member 75 to hold the plunger member in its first or inoperative position as illustrated in FIG. 4. Sealing members, such as O-rings 97, are sealingly engaged around the shank 94 where it passes through the longitudinal passage 90 to prevent well fluids from leaking across the plunger member 75. A compression spring 98 engaged between a projecting shoulder 99 on the shank 94 and an upwardly-facing shoulder 100 in the reduced bore portion 92 holds the shank 94 in operative engagement with the operator 95 of the switch 76 whenever the plunger member 75 is explosively impelled to its second position.

In operation, an explosive dynamic force below and proximate to the outwardly-directed face 101 of the piston portion will fracture the shear pin 96 holding the plunger rnember in its first position and explosively impel the plunger member upwardly into its second position where the upper end 102 of the shank 94 will engage the operator of the switch 76 to operate it.

Either a small passage through the piston portion 93 or a small annular clearance 103 is provided between the piston portion 93 and the inner wall 104 of enlarged bore portion 91 so that should well fluids leak into charge housing 79 below the outwardly-directed piston face 101, the fluids will gradually fill the space above piston portion 93 to equalize the pressure across the piston portion. It will be realized, of course, that in this event the hydraulic forces directed against the plunger member 75 will effectively act only on the cross-sectional area of the shank portion 94 since the hydraulic pressure will be applied equally on both sides of the piston portion 93. Accordingly, the diameter of the shank portion 94 is purposely reduced and the shear pin 96 is made of sufficient strength that the total expected hydraulic force that could be applied on this small effective area will be inadequate to fracture the shear pin and force the plunger member 75 upwardly to its operative position. Thus, in the event that well fluids do leak into a charge housing below an arming switch incorporating this feature, the plunger member will be positively restrained from connecting the blasting cap in the next perforating unit into the firing circuit. Furthermore, since arming of the perforating units is wholly dependent upon successive firing of each unit in turn, fleakage of well fluids into any intermediate unit will serve to effectively render all units thereabove inoperative as well. Thus, it will be appreciated that use of arming switches employing this principle of the invention will positively ensure against perforating off depth.

The self-contained switch 76 mounted on the upper end of the switch case 87 is disposed within that portion of the longitudinal bore 86 through the selective-firing head 74 which lies above the upper end of the switch case 87. Although any switching arrangement may be used, the switch 76 may be a single-pole single-throw switch connected in the manner illustrated in FIG. 1 or, if desired, a single-pole double-throw switch connected in the manner illustrated in FIG. 5. At the upper end of the selective-firing head 74 housing, the longitudinal bore 86 therethrough is proportioned to receive a conductor passage seal plug 105 to prevent fluids that might leak into the charge housing 78 above the firing head 74 from passing into the bore 86 and shorting the switch 76.

A conductive member 106 is received and extended through a longitudinally-extending passage 187 through the plunger member 75 in such a manner that the conductive member 186 is insulated from the plunger member 75. This conductive member 106, similar to the corresponding member used in the arming switch of FIG. 2, provides an electrical path through the arming switch 73 for connection to conductors 188, 109 above and below the arming switch.

In FIG. 5 a perforating apparatus 110 is schematically represented with a firing circuit which is polarity sensitive rather than current sensitive. A monocable 111, which has a central conductor 112 and an armor sheath 113, is connected at the surface of the ground to a control switch 114 connected in series with the central cable conductor 112, an ammeter 115, a polarity-reversing switch 116 and a source of DC. voltage, such as a battery 117. A potentiometer could also be added if further control was required. The polarity-reversing switch 116 is arranged in the conventional manner to selectively connect the positive side of the DC. source 117 to either the conductor 112 or the armor sheath 113 while respectively and simultaneously connecting the negative side of the DC. source to either the armor sheath or the conductor.

Disposed within carrier 118 are a number of electrically fired blasting caps 119-122 with filament igniters of equal current sensitivity for selectively dctonating a number of shaped charges (not shown). The blasting caps are in parallel with one another with one side of each igniter being connected to the reversing switch 116 through the cable sheath 113 and carrier 118. With the exception of blasting cap 119 which is fired first, the other end of each of the igniters is serially connected respectively to a rectifier or diode 123-125, each of which are con nected to the normally-open contact of a single-poie double-throw arming switch 126-128.

Blasting cap 119, which is in the lowermost perforating unit 12? to be fired first, is connected to the normallyclosed contact of arming switch 126 which has its normally-open contact connected to diode 123 and its moving contact or switch arm connected to the normallyclosed contact of arming switch 127. The moving contact or switch arm of arming switch 127 is, in turn, connected to the normally-closed contact of arming switch 123 and the normally-open contact of switch 127 is connected to diode 124. Arming switch 123 of the uppermost perforating unit 132 in the carrier has its normallyopen contact connected to diode 125 and its switch arm connected to the central conductor 112 of monocable 111.

It will be noted that diodes 123-125 are alternately oriented so that alternate diodes 123 and 125 are responsive to voltage of one polarity and intermediate diode 124 is responsive only to voltage of the opposite polarity. As previously explained, with this arrangement, voltage of a first polarity is applied to detonate the first shaped charge in the tool, which explosion closes the next adjacent arming switch. Thus, before the next explosive charge can be detonated, the polarity of the firing voltage must first be reversed.

From the foregoing description, it will be appreciated that if the negative side of the DC. source 117 is coupled to the armor sheath 113 and the positive side is coupled to the central conductor 112, whenever voltage is passed through the lowermost blasting cap 119 and detonate-s the shaped charge associated with that blasting cap, the explosion will actuate the arming switch 126 of the next perforating unit 130. Actuation of the arming switch 126 will then connect the diode 123 associated with that switch into the firing circuit in preparation for detonation of perforating unit 130. Because of the polarity orientation of diode 123, however, it will be realized that when arming switch 126 is actuated by the firing of the adjacent and lowermost perforating unit 129, the firing current will not pass next successive arming switch 127, it is necessary that the operator first reverse the polarity of the firing voltage before the next perforating unit 131 can be fired. Firing of the remaining perforating units continues in like manner until all units have been fired.

Turning now to FIG. 6, an alternate arrangement is shown of charge housings and selective-firing heads employing the arming switch of FIG. 7 which incorporates the principles of the invention. A perforating unit 133 comprised of a charge housing 134 and a selective-firing head sub or housing 135 is shown in full with portions of a second selective-firing head 136 immediately above and another charge housing 137 immediately below. A shaped charge 138 is mounted adjacent to an explosion port 139 in the charge housing 134 which is closed by closure 140. A blasting cap 141 dependently attached to a length of detonator cord 14?; connected to the shaped charge 138 in the well-known manner is disposed adjacent to an access port 143 in one side of the selective-firing head 135. Here again, electrical terminals 144, 145 to the blasting cap 141 may be left unconnected until such time that the perforating tool is completely assembled.

Although the electrical connections are shown to be conventional crimped connectors 146, 14-7 it will be appreciated that any other electrical terminals can be employed so long as they are accessible through the access port 143 in the side of the selective-firing head housing or sub 135.

The arming switch 148 depicted in FIG. 6 is shown in detail in FIG. 7. The switch assembly 149 includes an elongated tubular case 150 having an enlarged-diameter cup-shaped end member 151 and is provided with an O- ring 152 for sealing reception in a complementarilyshaped port-ion 153 of the axial bore 154 through the firing head 155. A. locking nut 156 threadedly secures the case 151) and end member 151 within the firing head housing.

An axial bore extending through the switch case 150 and end member 151 includes a reduced diameter bore portion 157 and enlarged bore portion 153 in the case 151) and an enlarged bore portion 159 in the cup-shaped end member 151. A plunger member 160 is disposed Within the bores 157-159 and includes a piston end-portion 161 partially received in the enlarged bore portion 159 of the cup-shaped end member 151 and a reduced-diameter end or shank 162 which projects through the intermediate reduced bore portion 157 and extends into the en larged bore portion 158 at the opposite end of the case. An O-ring 163 is sealingly engaged between the plunger shank 162 and the wall of the reduced bore portion 157 to fluidly seal the plunger member relative to the switch case.

The piston head 161 of plunger member 160 has an integrally-formed annular flange 164 which is initially disposed between the outer end 165 of cup-shaped member 151 and lock nut 156. This flange 164 is sized to yieldably restrain the plunger member 160 in its first or inoperative position until an explosive force acting on the outwardly-directed face 166 of the piston portion shears .the flange 164 and drives the plunger 1'60 inwardly to an operative second position.

As has been previously described in detail vwith respect to the embodiment of FIG. 4, the piston 161 of the FIG. 7 embodiment is also pressure equalized in the same manner by providing small passages 167 drilled through the piston head 161. Here again, the diameter of the shank 162 is purposely reduced and the shear flange 164 is of sufiicient strength that the hydraulic force applied by leaking well fluids on this small effective cross-sectional area will be inadequate to fracture the shear flange 164 and force the plunger 160 to its second position. It is believed that further detail is not necessary since this feature was fully explained above.

In the preferred form of the arming switch 149, the outermost portion 168 of the shank 162 is of a smaller diameter than the inner portion 169 of the shank which is adjacent to the piston end-portion 161. A short electrically-conductive sleeve member 170 is slipped over this outer or smaller diameter portion 168 of the shank 162 and arranged with its lower end engaged by the shoulder -171 formed where the smaller outer shank portion 168 joins the larger shank portion 169. The sleeve 170 is electrically insulated from the shank 162, as will be later described in detail, and is terminated at a point remote from the upper or outer extremity of the shank.

A pair of spaced-apart electrical contacts 172, 173 are disposed in the enlarged bore portion 159 within the switch case 151] and are arranged so that the outermost contact 173 is engaged with the uninsulated outermost tip 174 of the plunger shank and the innermost contact 172 is either lightly engaged or nearly so with the plunger shank intermediate the shank tip 174 and the upper end of the conductive sleeve member 170.

In the preferred form of the arming switch, these contacts are annular copper rings disposed around the smaller diameter shank portion 168 with the outermost contact 173 having its central hole flared upwardly, as at 175, to provide a better electrical contact with the uninsulated tip 174. The rings are firmly held in place by a series of insulator rings 176, 177 and 178 placed on each side of the two copper contact rings 17-2, 173 and received inside a tubular insulating sleeve 1179 which is snugly received within the upper enlarged bore portion 158 of the switch case I150. If desired, a diode 180 may be disposed within the insulating sleeve 179 and the entire cavity filled with a resilient plastic insulating material 181 to insulate the leads and components as well as to seal the switch unit and shock mount the components.

It is preferred that the plunger member 160 be constructed from aluminum and that, as indicated by the dotted markings in FIG. 7, the outer surfaces thereof be hard-anodized preferably to a depth of about 0.00 l0.002 inch. it has been found that for the relatively low voltages customarily used, such hard-anodizing provides an excellent electrical insulation on the surfaces of the plunger while still allowing transmission of electricity from the uninsulated tip portion 174 through the body of the plunger member 160, Thus, it will be appreciated that when the plunger 160 is in the inoperative position shown in FIG. 7, electricity can pass along conductor 182, through contact ring 173 to the outermost tip 174 of the shank .16 2, and on through the plunger member 160 to an electrical connector 18C) engaged in the out= wardly-directed face 166 of the piston end. It will be further appreciated that when the plunger 160 is explosively impelled upwardly to its second or operative position, the conductive sleeve 1170 will be driven through both contact rings 172, 173 to electrically connect these rings and allow current to flow through the switch conductors 182, 184, diode 180 and conductor i185 to complete the firing circuit to the blasting cap associated with the switch. At the same time, since the sleeve 170 is insulated from the plunger shank 162, the plunger member 160 is removed from electrical contact with the other elements thereby breaking the firing circuit to perforating units below the switch. It will be appreciated that the frictional engagement of .the sleeve 1170 with contacts 17-2, 173 will be sufiicient to hold the plunger member in its operative position even if no well fluids enter the lower adjacent perforating unit.

Although the above-described anodizing is preferred, it would be equally feasible to use other arrangements and materials which would give the equivalent results. For example, the insulated surfaces could be obtained by varnishing the plunger, by nitriding or phosphating,

or by coating the plunger with Teflon or other similar electrically-nonconductive materials. Furthermore, simple spring contacts could be used for the ring contacts.

As seen in FIG. 7, the outwardly-directed face 166 of the piston end-portion 161 is preferably provided with U a recess 186 which is adapted to receive a banana-plug connector 183 carrying a disc 187 on its outer end. The disc 187 is of a larger diameter than the piston end 16 1 so the explosive force will be initially applied against a larger area than that of the piston alone.

it will be appreciated that in all three embodiments of the arming switches as described, when well fluids enter a perforating unit .that has been fired, the hydrostatic pressure will hold the piston member of the switch unit closed to maintain the electrical connection. If, however, no fluids entered, the concussion forces of each explosion would close the arming switch and the switch would be held closed only by the various members disclosed as restraining the switches in their operative position.

It is also contemplated that a particular tool configuration might result in locating a shaped charge too far from the arming switch of the adjacent perforating unit and that if the perforating tool is being operated in a dry well bore, the concussion force might be ineffective to positively close the arming switch. In such cases, it is preferred to extend the blasting cord upwardly into close proximity of the piston face to utilize the explosive force of the blasting cord to close the switch.

While particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

What is claimed is:

1. In combination with a sealed perforating apparatus having separate chambers each containing electricallyactuated explosive perforating means, said apparatus having a bore interconnecting said chambers, electrical switching means for establishing an electrical circuit to one of said perforating means comprising: a piston member having enlarged and reduced portions with said reduced portion being arranged for travel in said bore between spaced longitudinal positions; first and second electrical contacts opcratively arranged to be open in one of said piston positions and to be closed in the other of said positions; means for fluidly sealing said reduced piston portion relative to said bore; means restraining said piston member in said one position and responsive to dynamic explosive pressures in one of said chambers against said enlarged piston portion to release said piston member and permit travel of said piston member to said other position; and means preventing release of said restraining means whenever liquid fills said one chamber including flow restriction means in fluid communication between said one chamber and said bore intermediate of said enlarged piston portion and sealing means.

2. In combination with a pressure-sealed perforating apparatus having separate chambers each containing an electrically-fired explosive charge, an arming switch comprising: a switch case sealingly received in said perforating apparatus between said chambers, said case having a longitudinal bore therethrough; a piston member having a reduced portion received in said bore and arranged for travel from a first position to a second position longitudinally spaced relative to one end of said case, said piston member having an enlarged portion with an outwardly-directed face responsive to explosive forces exterior of and approximate to said one case end for impelling said piston member inwardly from said first position to said second position; first restraining means for yieldably holding said piston member in said first position, said first restraining means being ineffective to prevent travel of said piston member to said second position in response to such explosive forces; second restraining means for holding said position member in said second position whenever said piston member reaches said second position; means for fluidly seaiing said reduced piston portion relative to said bore; first and second normally-open electrical contacts operatively spaced and arranged for electrical interconnection whenever said piston member reaches said second position; and means preventing travel of said piston member Whenever the chamber proximate to said case end fills with liquid including flow restriction means in fluid communication between said one chamber and said bore intermediate of said enlarged piston portion and sealing means.

3. As a sub-combination with perforating apparatus adapted to be sealed for confining electrically-fired expiosive charge means in said apparatus, an arming switch comprising: a switch case adapted to be sealingly mounted on said perforating apparatus, said case having a longitudinal bore therethrough; an electrical switch having an actuator and adapted for mounting adjacent to one end of said case; a piston member having a reduced portion received in said bore and arranged for travel inwardly from a first position to a second position longitudinally spaced relative to the other end of said case, said piston member having an enlarged portion with an outwardly-directed face responsive to explosive forces exterior of and proximate to said other case end for impelling said piston member inwardly from said first position to said second position; means releasably restraining said piston member in said one position and releasable in response to such explosive forces; means for fluidly sealing said reduced piston portion relative to said case; means for actuating said switch including an actuating member extending from said piston member and operatively arranged to engage said switch actuator and operate said switch whenever said piston member reaches said second position; and flow restriction means in fluid communication between said bore intermediate of said sealing means and said enlarged piston portion and said other case end.

4. The apparatus of claim 3 in which said piston member has a substantially longitudinal passage extending therethrough, a conductive member sealingly received in said passage and electrically insulated from said piston member, said conductive member being adapted for connection of electrical conductors to each end thereof.

5. In combination with a pressure-sealed perforating appartus to be operated in a well bore having a given hydrostatic pressure therein, said apparatus having separate chambers each containing an electrically-fired explosive charge, an arming switch comprising: a switch case adapted to be sealingly mounted between said chambers, said case having a longitudinal bore therethrough, said bore having an enlarged portion and reduced portion; an electrical switch having an actuator and adapted for mounting adjacent said reduced bore portion; a plunger member having an enlarged piston portion received in said enlarged bore portion and a reduced shank portion received in said reduced bore portion, said plunger member being arranged for travel inwardly from a first position to a second position longitudinally spaced relative to the other end of said case in response to explosive forces exterior of said other case end in one of said chambers said shank portion being constructed and arranged to engage said switch actuator for operating said switch whenever said plunger member reaches said second position; restraining means for yieldably holding said plunger member in said first position, said restraining means being ineffective to prevent travel of said plunger member to said second position in response to such explosive forces; means for fluidly sealing said shank portion relative to said case; and means preventing travel of said plunger member whenever liquid fills said one chamber including flow restriction means in fluid communication between said one chamber and said bore intermediate of said piston portion and sealing means.

6. The apparatus of claim 5 in which said restraining means is a frangible member engaged between said plunger member and said case and arranged to fracture for releasing said plunger member only upon application of a pressure force to said piston portion in excess of the product of the cross-sectional area of said shank portion multiplied by the known hydrostatic pressure of the well bore.

7. For use in a pressure-sealed perforating apparatus having electrically-fired explosive means in the bore of the apparatus, an arming switch comprising: a piston member received in said bore and arranged for longitudinal travel from a first position to a second position in response to explosive forces; means for fluidly sealing said piston member relative to said bore including sealing means encircling said piston member and sealingly en gaged therebetween; first restraining means for yieldably holding said piston member in said first position, said first restraining means being ineffective to prevent travel of said piston member to said second position in response to an explosive force; second restraining means including a resilient finger-lil e member projecting angularly from said piston and extending toward one side of said bore and stop means on said side of said bore arranged to engage and retain the free end of said resilient member whenever said piston member reaches said second position for holding said piston member in said second position; a first contact carried by said piston member;

and a second contact on the wall of said bore and arranged relative to said first contact for engagement thereby when said piston member reaches said second position, one of said contacts being electrically insulated from said apparatus.

8. The arming switch of claim 7 in which said piston member has a substantially longitudinal passage extending therethrough, a conductive member sealingly received in said passage and electrically insulated from said piston member, said conductive member being adapted for connection of electrical conductors to each end thereof.

References Cited by the Examiner UNITED STATES PATENTS 1,641,483 9/1927 Greene 102-21.8 2,335,073 11/1943 Martin et al. 200-82 2,543,823 3/1951 Barry. 2,745,345 5/1956 Sweetman 102-218 3,010,396 11/1961 Coleman l0221.6 3,031,548 4/1962 Robinson 200-82 X BENJAMIN A. BORCHELT, Primary Examiner.

V. R. PENDERGRASS, Assistant Examiner. 

1. IN COMBINATION WITH A SEALED PERFORATING APPARATUS HAVING SEPARATE CHAMBERS EACH CONTAINING ELECTRICALLY ACTUATED EXPLOSIVE PERFORATING MEANS, SAID APPARATUS HAVING A BORE INTERCONNECTING SAID CHAMBERS, ELECTRICAL SWITCHING MEANS FOR ESTABLISHING AN ELECTRICAL CIRCUIT TO ONE OF SAID PERFORATING MEANS COMPRISING: A PISTON MEMBER HAVING ENLARGED AND REDUCED PORTIONS WITH SAID REDUCED PORTION BEING ARRANGED FOR TRAVEL IN SAID BORE BETWEEN SPACED LONGITUDINAL POSITIONS; FIRST AND SECOND ELECTRICAL CONTACTS OPERATIVELY ARRANGED TO BE OPEN IN ONE OF SAID PISTON POSITIONS AND TO BE CLOSED IN THE OTHER OF SAID POSITIONS; MEANS FOR FLUIDLY SEALING SAID REDUCED PISTON PORTION RELATIVE TO SAID BORE; MEANS RESTRAINING SAID PISTON MEMBER IN SAID ONE POSITION AND RESPONSIVE TO DYNAMIC EXPLOSIVE PRESSURES IN ONE OF SAID CHAMBERS AGAINST SAID ENLARGED PISTON PORTION TO RELEASE SAID PISTON MEMBER AND PERMIT TRAVEL OF SAID PISTON MEMBER TO SAID OTHER POSITION; AND MEANS PREVENTING RELEASE OF SAID RESTRAINING MEANS WHENEVER LIQUID FILLS SAID ONE CHAMBER INCLUDING FLOW RESTRICTION MEANS IN FLUID COMMUNICATION BETWEEN SAID ONE CHAMBER AND SAID BORE INTERMEDIATE OF SAID ENLARGED PISTON PORTION AND SEALING MEANS. 